A component for machine structural use that is used for an automobile transmission and the like, such as a gear and a pulley for a belt-type continuously variable transmission (CVT), is usually subjected to surface hardening treatments from the viewpoint of improving bending fatigue strength, pitting strength, and wear resistance. There are typical surface hardening treatments such as “carburizing and quenching”, “induction quenching”, and “nitriding”.
Among the treatments mentioned above, the “carburizing and quenching” is a treatment in which a low carbon steel is generally used; and in the said treatment, C is caused to invade and diffuse in an austenitic region of a high temperature not less than the Ac3 point, and thereafter quenching is carried out. The “carburizing and quenching” has an advantage of attaining a high surface hardness and a large effective case depth, but this treatment is accompanied by phase transformation; and thus in the said treatment, there is a problem that the heat treatment distortion becomes large. Therefore, in the case where the high component accuracy is required, it is necessary to carry out a finish working, which is grinding, honing and so on, after the “carburizing and quenching”. In addition, the “carburizing and quenching” has a problem that a so-called “abnormal carburized layer”, which is an intergranularly oxidized layer, non-martensitic layer and so on, produced on the outer layer becomes a start point of failure such as bending fatigue failure, and the fatigue strength is deteriorated.
The “induction quenching” is a treatment in which a steel is rapidly heated to an austenitic region of a high temperature not less than the Ac3 point and thereafter quenched. The “induction quenching” has an advantage that the effective case depth can be controlled with relative ease, but this treatment is not a surface hardening treatment in which C is caused to invade and diffuse like the carburizing treatment. Therefore, in the case of the “induction quenching”, in order to attain necessary surface hardness, effective case depth and core hardness, a medium carbon steel, which has a higher C content as compared with a steel for carburizing treatment, is generally used. However, as for a starting material, the medium carbon steel has a higher hardness than the low carbon steel; and thus there is a problem that steels for the said “induction quenching” are inferior in machinability. In addition, with regard to the “induction quenching”, a high frequency heating coil must be prepared for each component.
In contrast, the “nitriding” is a treatment in which N is caused to invade and diffuse at a temperature of about 400 to 550° C. not more than the Ac1 point, and thereby a high surface hardness and a proper effective case depth are attained. In the case of the “nitriding”, as compared with the “carburizing and quenching” and the “induction quenching”, the treatment temperature is low; and therefore the said “nitriding” has an advantage that the heat treatment distortion is small.
In addition, in the “nitriding”, the “nitrocarburizing” is a treatment in which N and C are caused to invade and diffuse at a temperature of about 500 to 650° C. not more than the Ac1 point, and thereby a high surface hardness is attained. This treatment is suitable for mass production because the treatment time is as short as several hours.
Furthermore, along with the trend toward the reduction in greenhouse gas with the recent restraint of global warming being a background, it has been demanded that a process in which a steel is held at a high temperature, such as “carburizing and quenching”, be reduced. Therefore, the “nitriding” is a treatment responding to the demand of the day.
Unfortunately, the conventional steel for nitriding has problems described in the following <1> to <3>.
<1> The “nitriding” is a surface hardening treatment in which quenching from an austenitic region of a high temperature is not performed, that is to say, it is a surface hardening treatment in which strengthening accompanied by the martensitic transformation cannot be performed. Therefore, in order to provide a nitrided component with the desired core hardness, it is necessary to contain a large amount of alloying elements, and thus it is necessary to perform forming by hot forging or the like because it is difficult to perform forming by cold forging.
<2> As for a typical steel for nitriding, the “Aluminum Chromium Molybdenum Steel (SACM645)” specified in JIS G 4053 (2008) is available. With regard to the steel of this type, unfortunately, although a high surface hardness can be attained because Cr, Al and the like produce nitrides near the surface, a high bending fatigue strength cannot be attained because of a shallow effective case depth.
<3> In the nitrocarburizing of the nitriding, a component is held in a temperature range of about 500 to 650° C. for several hours; and thus the core of the said component is liable to be softened by tempering. As a result, for a component to which a high contact pressure is applied, plastic deformation is easily produced in the core, and the contact surface is depressed and deformed.
Accordingly, in order to solve the problems mentioned above, for example, the Patent Literatures 1 and 2 disclose techniques concerning the “nitriding”.
The Patent Literature 1 discloses a “steel for nitrocarburizing excellent in cold forgeability” having an objective of providing a steel for nitrocarburizing that has a hardness after rolling of 200 or less in Vickers hardness, and is excellent in nitrocarburizing property and cold forgeability. The aforementioned “steel for nitrocarburizing” consists, by mass percent, of C: 0.05 to 0.25%, Si: 0.50% or less, Mn: 0.55% or less, Cr: 0.50 to 2.00%; V: 0.02 to 0.35%, and Al: 0.005 to 0.050%, and further according to need Nb: 0.02 to 0.35%, with the balance being Fe and impurities.
The Patent Literature 2 discloses a “method for producing a nitrided component” by which a hard surface hardened layer, a large effective case depth, and the necessary core hardness can be obtained and in which the amount of machining such as cutting can be reduced. The aforementioned “method for producing a nitrided component” provides a technique in which a steel material having a chemical composition consisting, by mass percent, of C: 0.10 to 0.40%, Si: 0.10 to 0.70%, Mn: 0.20 to 1.50%, Cr: 0.50 to 2.50%, and V: 0.05 to 0.60%, and further according to need one or more of Al, Mo, Ti, Nb, Ta, B, S, Pb, Te, Se, Ca, Bi and Sb, with the balance being substantially Fe is subjected, before nitriding treatment, to heat treatment in which the precipitation of V is controlled, subsequently being subjected to a cold working, and is further subjected to a nitriding treatment.